Trimanganese tetraoxide is converted to a lithium manganese oxide by being mixed with a lithium material and another metal material, followed by firing. For example, rhombic LiMnO2 obtained by mixing and grinding trimanganese tetraoxide and lithium hydroxide, followed by firing, has been reported (Patent Document 1). Further, a lithium nickel manganese cobalt composite oxide obtained by forming trimanganese tetraoxide, lithium carbonate, cobalt oxyhydroxide, nickel hydroxide and the like into a slurry, and wet grinding the slurry, followed by firing, has been reported (Patent Document 2).
In such processes for producing a lithium manganese oxide using trimanganese tetraoxide, a so-called necking phenomenon i.e. a phenomenon such that particles of the lithium manganese oxide are fused with one another during firing is likely to occur. Due to the necking phenomenon, the lithium manganese oxide obtained by such a production process tends to be non-uniform in its particle size, particle shape, etc.
In order to make such a non-uniform lithium manganese oxide be uniform, it is necessary to grind or pulverize the lithium manganese oxide after firing (Patent Documents 1 and 2).
However, in a case where grinding or pulverization is carried out after firing, not only the production cost of the lithium manganese oxide is increased but also problems are pointed out that abrasion powder from the grinding medium may be included (for example, Patent Document 3).